1. Field of the Invention
The present invention relates an ink jet recording apparatus wherein an image is recorded by discharging ink from an ink discharge port toward a recording medium.
The present invention also relates to a device for controlling a temperature of ink jet recording means, which utilizes a heat exchanging means such as a heat pipe acting as a means for adjusting a temperature of an ink jet recording head.
The present invention is effective for an ink jet recording apparatus, preferably full color ink jet recording apparatus which has one or more recording means of full-line type that can record along a maximum width of a recording medium.
2. Related Background Art
In the past, image recording methods utilizing thermal energy, such as heat sensitive recording methods, heat transfer recording methods or the like have widely been used with facsimiles, copying machines and the like, since they are highly reliable and provide good quality of an images.
Recently, in order to obtain higher recording speed, higher reliability and a better image, interest has been directed to a so-called ink jet recording apparatus wherein a desired image is formed by discharging ink by the use of thermal energy generated in response to a predetermined recording signal.
In such apparatus wherein the desired image is recorded by utilizing thermal energy, in order to achieve the high speed recording, a so-called recording head of full-line type which is designed to record the image by providing a plurality of heating elements along the whole width of a recording medium has been proposed.
If the image is formed (as a solid recording) on a partial area of the recording medium as shown in FIG. 1 by using a recording head of the full-line type, the temperature of a portion of the full-line type recording head which corresponds to the solid recording area will be increased to generate uneven distribution of temperature in the recording head as shown in FIG. 2A(1). Such uneven temperature distribution in the recording head causes overheating of some of the heat generating or heating elements and/or changes in the viscosity of ink, and, for example, if a half-tone recording area is then formed, the image recorded in the half-tone recording area will have the dispersion in density as shown in FIG. 2B(4), thus worsening the quality of the image.
In order to solve the problem regarding the occurrence of such local increase in temperature or uneven distribution of temperature, some of the inventors have proposed a technique wherein a heat exchanging means such as a heat pipe is attached to an ink jet recording head to improve such problem, as described in U.S. patent application Ser. No. 07/822,333, having an effective filing date of Dec. 28, 1989.
Although such proposed technique wherein the heat pipe is merely attached to the recording head provides a satisfactory temperature adjustment during the recording operation at a low speed, it does not attain the temperature adjustment satisfactorily during the high speed recording operation.
That is to say, in the proposed technique, by attaching the heat pipe to the recording head as mentioned above, the dispersion in temperature as shown in FIG. 2B(4) could be avoided during the low speed recording operation; however, particularly, if the recording of the image was effected on A4 size sheets by utilizing the recording head of full-line type at a high speed (for example, 40 sheets per minute), an adequate temperature adjustment could not be attained.
For example, when the image as shown in FIG. 1 was recorded on 60 sheets continuously by utilizing the recording head including the heat pipe attached thereto, the temperature of a portion of the recording head which corresponds to the solid recording area A was increased up to 50.degree. C. as shown in FIG. 2A(2) even if the temperature of the recording head was adjusted to 40.degree. C. In this case, the recording density in the half-tone recording area created the dispersion in density of 0.10 at the maximum as seen from FIG. 2B(5), thus worsening the quality of the image.
In consideration of the above results, the inventors have proposed a recording head unit wherein both a heat pipe 2 and means for keeping the temperature of the heat pipe 2 even or uniform are associated with a recording head 1 of full-line type. In this arrangement, the recording head 1 has a plurality of orifices disposed along the width of the head and facing toward a recording medium A and further has elements for applying thermal energy to ink supplied to each orifice. Further, in order to balance or equilibrate the temperature through the whole recording head, the heat pipe 2 is provided with heating means 3, heat discharging and cooling means 4, temperature detecting means 5 and the like (which act as means for adjusting the temperature of the heat pipe 2) so that the temperature of the heat pipe is adjusted by selectively activating the heating means 3 or the cooling means 4 through control means 6 on the basis of a detection result from the detecting means 5. Accordingly, the heat exchange between the recording head and the heat pipe (the temperature of which is adjusted) is permitted, thereby providing the even temperature distribution through the whole recording head.
However, in a multi-color recording apparatus, a plurality of such recording head units must be arranged in parallel along the recording medium A. Consequently, the whole apparatus becomes bulky, complicated and expensive. In particular, the heat discharging and cooling means 4 becomes bulky and expensive since it includes heat discharge fins and a blower, with the result that a distance between the adjacent recording head units must be increased. Thus, there arise problems that it is difficult to obtain the registration (coincidence in positions where the colors are superimposed) at high accuracy and the whole apparatus becomes bulky.
In consideration of the above problems, as shown in FIG. 4, in the multi-color recording apparatus, the provision of a common positive heat discharging means 4 including a heat discharging means 17 and a cooling means 4b (for cooling the discharging means 17) associated with extension portions 2a-l-2d-l of heat pipes 2a-2d extending outside of the recording area has been proposed. In this arrangement, the heat pipes 2a-2d are associated with the recording heads 1a-1d, respectively, and the heat pipes 2a-2d include heating means 3a-3d and temperature detecting means 5a-5d, respectively, in the recording area. With this arrangement, since the distance between the recording head units can be reduced, the above registration can be effected conveniently.
However, in this case, if the particular recording head among the recording heads 1a-1d is used continuously (i.e., for example, the recording operation is performed by using only black ink), since the control signal of the cooling means 4b is controlled on the basis of the detected temperature of said recording head, the heat pipes corresponding to the non-used recording heads will also be cooled. Consequently, since the temperature detecting means associated with the non-used recording heads detect the reduction in temperatures of the corresponding recording heads and send the detected signals to the control means 6, the heating means corresponding to the non-used recording heads are also activated, with the result that the cooling and the heating are simultaneously effected totally, thus generating the considerable loss of the electric power.
On the other hand, as shown in FIG. 4, in the recording apparatus wherein the plurality of recording heads are arranged in parallel to form a color image, as mentioned above, a distance d between the recording heads is required to be reduced as much as possible within a range that any recording head is not influenced by the adjacent recording head. This requirement is desired in view of the improvement in the above registration (position alignment between the recording head and/or position alignment between the recording areas) and the compactness of the recording system, and, thus, the shorter distance d between the recording heads is preferable. Further, if the color image is recorded by reading the color image, the longer the distance d between the recording heads, the more the memory amount for storing the image data is increased, which leads to increasing cost of the recording system. This is also one of the reasons that the distance d between the recording heads must be reduced.
Further, in the recording apparatus wherein the heat pipe is attached to the recording head, the heat conveying capacity of the heat pipe must be considered. That is to say, since the heat conveying capacity of the heat pipe has a certain limitation, in order to obtain the adequate thermal feature, it is necessary to use a heat pipe which can transmit more calories than the maximum calories that can be transmitted from the recording head to the heat pipe or from the heat pipe to the recording head.
The limitation of the heat conveying capacity varies in accordance with a diameter R of the heat pipe, and the larger heat conveying capacity is obtained as the diameter of the heat pipe is increased.
Accordingly, in the recording apparatus wherein the plurality of recording heads are arranged as shown in FIG. 4, inconsistent requirements (i.e., the distance d between the recording heads must be reduced, and at the same time the diameters of the heat pipes must be increased) are present.
In order to alleviate such inconsistency, it can be considered that each heat pipe is designed to have a flat configuration as shown in FIG. 4 so that a thickness of the heat pipe is smaller than the distance d between the recording heads. In this case, however, an area of each heat pipe extending outwardly of a contacting area between the heat pipe and the corresponding recording head had the poor heat exchanging ability. That is to say, as shown in FIG. 4, since the whole heat pipe was designed to have the flat configuration, the maximum heat conveying capacity of each heat pipe was reduced by 10% in comparison with that of the cylindrical heat pipe. The reason therefor will be derived from the fact that the conveying capacity of the vapor of the operating liquid generated in a heat input area or contacting area between the heat pipe and the corresponding recording head is reduced. Such reduction in the heat conveying capacity causes no problem in the heat input area. However, the heat discharging area is greatly influenced by such reduction in the heat conveying capacity, which results in the reduction in the heat exchanging ability. The reason therefor will be derived from the fact that the vapor of the operating liquid generated in the input area cannot be effectively transmitted to the whole heat discharging area due to the reduction in the heat conveying capacity.
Now, it is considered that heat discharging fins are formed by press fitting a plurality of aluminium plates each having a thickness, for example, of about 0.3 mm, as shown in FIG. 4.
This method is preferable to provide the heat discharging means in the point that surface areas of the whole fins can be increased and the cost is inexpensive. However, when the heat pipes are designed to have the flat configuration, the dispersion will occur in surface shapes of the heat pipes. If such dispersion in the surface shapes is great, it was difficult to fix the fins by the press fit to obtain the stable thermal contacting condition between the heat pipes and the fins.
By the way, from another point of view, an ink jet recording apparatus utilizing a heat pipe is disclosed in the Japanese Patent Publication No. 62-55990. This Publication teaches an arrangement wherein the heat pipe is attached to a recording head of full-line type, for applying the heat generated in the recording head to a recorded recording medium to dry ink on the recording medium. This known technique aims to utilize the waste heat of the recording head, which is quite different from the problem to be solved by the present invention. The heat pipe of this known technique has a uniform cross-section and is designed in an inverted C-shaped configuration for closely contacting with the whole recording area of the recording head to absorb the waste heat from the recording head. Thus, this publication merely teaches the re-use of heat to the recorded image, but does not refer to the above-mentioned problems regarding the recording head itself in the recording process.